1. Field of the Invention
The present invention relates to a printing apparatus having a print head mounted therein, and in particular, to a printing apparatus having a print head mounted therein and also having heating means such as a heater for heating the printing medium to dry a printing liquid such as ink.
2. Description of the Related Art
Conventional ink jet printing apparatuses eject ink droplets as a printing liquid from a print head directly onto a printing medium and deposit the ink droplets on surfaces of the printing medium for printing thereon. Because of their simple mechanisms for printing, these ink jet printing apparatuses are frequently used in printers, copiers, or the like. Furthermore, in many of these ink jet printing apparatuses, the printing liquid deposited on the printing medium is naturally dried to fix print images on the surfaces of the media.
Further, in these ink jet printing apparatuses, thermal drying means of the printing liquid is effective means for preventing the printing liquid from bleeding at edges of a print image on a printing medium, allowing the print image to be fixed quickly to the printing medium, improving the density of the print image, or the like. If such means is used, the ink jet printing apparatus is provided with heating means such as a heater for heating portions of the printing medium which are near the print head.
In an ink jet printing apparatus mounted with a print head, a surface of the print head is located opposite to a printing medium such as a print sheet and is called a “face”. The face of the print head has ejection ports formed therein to eject the printing liquid onto the printing medium therethrough. Thus, the printing liquid is ejected onto the front surface of the printing medium through the ejection ports of the print head to print onto the printing medium. Further, in the case of thermally drying the printing liquid, some ink jet printing apparatuses are provided with a heater or the like which heats portions of the printing medium which are near the print head. A typical location where the heater or the like is provided includes a front surface of a platen corresponding to the back surface of the printing medium, located opposite to the face of the print head, and the bottom of the platen. Another proposed location for the heater or the like is one immediately adjacent to the print head which corresponds to the top of the surface of the printing medium, which has just been printed by the print head. Moreover, such a heater or the like simultaneously heats portions of the printing medium which are near the portions to be printed.
The inventors have experimentally found that while the printing liquid ejected from the print head is permeating through the printing medium after being deposited on the printing medium, heating carried out as early as possible more effectively prevents the printing liquid from bleeding at the edges of the print image on the printing medium, allows the print image to be more quickly fixed by facilitating drying and permeation, and concentrates the color material of the printing liquid in the vicinity of the front surface of the printing medium in order to improve the density of the print image. When the color material of the printing liquid concentrates in the vicinity of the front surface of the printing medium, the amount of color material of the printing liquid present in a deep layer of the printing liquid which is close to the back surface of the printing medium decreases to avoid making the print image visible through the back surface of the printing medium, thereby improving the practicality of double-side printing These effects are marked particularly if the heater is mounted near the portions of the printing medium to be printed.
However, such an arrangement of the heater is not preferable because a heating section is located near to the print head, which may thus be heated. In particular, if the heater is mounted on the front surface of the platen corresponding to the back surface of the printing medium, located opposite to the face of the print head, or is provided at the bottom of the platen, the time required after the printing liquid from the print head has been deposited on the printing medium and before it is fixed is reduced to improve a thermal fixation effect, but the print head is heated by the heater, located immediately adjacent to the face of the print head. Thus, this configuration is more undesirable. Compared to the case in which the heater is mounted immediately adjacent to the print head, that is, above the front surface of the printing medium, which has just been printed, the case is particularly preferable in which the heater is mounted on the front surface of the platen corresponding to the back surface of the printing medium, located opposite to the face of the print head, or is mounted at the bottom of the platen, because the time required after the printing liquid from the print head has been deposited on the printing medium and before heating is started can be shortened, thereby eliminating the need to provide a space immediately adjacent to the print head in which the heater is mounted. If the amount of space that must be provided immediately adjacent to the print head can be reduced, then if a plurality of print heads are mounted, the interval between the print heads can be reduced, or the size of the printing apparatus can be reduced. But, the print head has ejection ports formed in the face thereof, so that a change in environment such as temperature or humidity is likely to affect ejection of the printing liquid. If the heater is mounted on the front surface of the platen corresponding to the back surface of the printing medium, located opposite to the face of the print head, or is mounted at the bottom of the platen, the printing liquid is prone to be heated at the ejection ports of the print head and have its physical properties changed before ejection. Consequently, there are possibilities that the printing liquid may be inappropriately ejected, the nature of the liquid on the printing medium may be changed, or the like. In particular, at the ejection ports of the print head, the heat causes the moisture of the printing liquid to be evaporated to be apt to dry the printing liquid for concentration.
The print head of the ink jet printing apparatus has a liquid channel formed therein and through which the printing liquid flows. When the printing head is heated, the temperature in the liquid channel rises to heat the printing liquid in the liquid channel. When the printing liquid in the liquid channel is thus heated, the viscosity of the printing liquid or the like may change, the physical properties of the printing liquid may change owing to alteration, or the like. Further, such heating may cause a gas dissolved in the printing liquid to be emitted from the printing liquid or may cause the printing liquid to be vaporized to generate bubbles in the printing liquid, thereby preventing the liquid from being delivered through the print head or being properly ejected.
On the other hand, the print head is often comprised of plastic or the like, which is not resistant to heat. This is because the print head can be inexpensively and easily made by entirely or partially forming its nozzle portion using plastic However, plastic or the like generally has low heat conduction and radiation, so that when exposed to high temperatures, the print head has its temperature easily increased. Further, if the print head is comprised of plastic having high heat conductivity and resistance, then material costs will increase or processing will be difficult. Accordingly, it is desirable to use a print head formed of the same material as that for conventional print heads. Further, circuit elements constituting an electronic circuit in the print head are also likely to be affected by heat. Consequently, if the print head is exposed to high temperatures and has its temperature increased, the print head member may be deformed or damaged, or the electronic circuit or the like in the print head may have its properties changed or may be damaged, thus irreversibly destroying the print head. In particular, if printing speed is increased, for example, if printing is carried out with a smaller number of passes, then the time spent in heating each portion of the printing medium is reduced, it results in the need to fix the print image to the printing medium by heating the printing medium at higher temperatures. Therefore, additional means is required for preventing the print head from being heated together with the printing medium even if the latter is heated at high temperatures, for allowing the printing medium to be efficiently heated with a reduced amount of energy, or the like.
Thus, in order to solve the above described problems of the conventional ink jet printing apparatuses, it is an object of the present invention to provide a printing apparatus in which when printing media are heated by using heating means such as a heater in order to fix a printing liquid to the printing media, an increase in temperature of the print head caused by such heating means is properly suppressed.
It is another object of the present invention to provide a printing apparatus that can efficiently fix a printing liquid to printing medium by effectively using heat from heating means such as a heater.